Light Pollution

[Rajiv Subrahmanyam]

I recently ran an experiment to judge the effect of light pollution. I captured the Rosette Nebula for 3 hours (1 minute subs), using a 50mm refractor, from 2 different locations:

1) Mountain View CA (Bortle 7) using SVBony CLS filter

2) Arnold CA (Bortle 2-3) with no filter

Post processing was not exactly the same, but I processed the images until I could get the best looking image I could in both cases.

The darker sky clearly resulted in a better image as expected, but I was still surprised at how little noise there was compared to the one shot from home. Has anyone else run similar experiments? Is there a more scientific way to conduct this experiment?

[Hy Murveit]

To take the opposing point of view...there's no doubt that darker skies result in better images, certainly with similar amounts of capture time. However, it's also true that, if you can set up in your backyard, you can usually capture for a longer time  than if you're traveling somewhere, setting up, staying awake there, shutting down, driving home. So, I'd say an alternative comparison might be to give extra time to the light-polluted skies. 

For instance, I can shoot a target for several nights (if the skies are clear) from home without much effort. If I drove down to Pinnacles, I might shoot for half a night (unless I rented a place down there). Perhaps compare 15 hours of light-polluted images vs 3 hours of dark sky shots? The exact numbers will vary according to your personal circumstances. 

I think the bottom line is that one can capture a Rosette Nebula image from the Bay Area similar in quality to your dark-sky Rosette given sufficient  Bay Area exposure time,  e.g. see the quality of the image in Francesco's last presentation. There are subtle details that probably may be too difficult to achieve from light polluted skies, but many reasonably good images can be captured and processed from the Bay Area.

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[Rajiv Subrahmanyam]

Good point Hy. It would be great to see how much longer it takes to get the same quality image from home, compared to the darker location.

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[Francesco Meschia]

That factor is the ratio between the two background sky brightnesses, which can be measured with an SQM. One magnitudine per square arcsecond of difference = a 2.5x factor. Two magnitudes = 6.3x. 5 magnitudes = 100x.

Francesco 

Sent from my iPhone

On Mar 6, 2022, at 13:42, Rajiv Subrahmanyam <rajiv....@gmail.com> wrote:



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[Rajiv Subrahmanyam]

Thanks Francesco!

I just looked up the SQM reading at lightpollutionmap.info for the two locations mentioned above.

My house: 18.58 mag/arcsec2

Arnold: 21.81 mag/arcsec2

which works out to (2.5 ^ 3.23 =) 19x as much time to get as good an image from here as there. In this case, I'd need 57 hours of exposure in MV to get as good as 3 hours in Arnold.

Interestingly, wikipedia says that the SQM range goes from 16 (most light polluted) to 22 (least light polluted), and Arnold is close to the theoretical minimum light pollution. However, per the bortle scale it is still 3, far from the minimum 1. Why the difference?

Rajiv

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[Francesco Meschia]

On the relationship between sky brightness and signal-to-noise ratio, I have prepared a video that simulates the difference between imaging a faint object from a dark site, and from a light-polluted one: https://youtu.be/WGjq6eGtkcU

Francesco

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[Hy Murveit]

Here's an interesting page on the definition of the Bortle Scale

http://www.cleardarksky.com/lp/GSSPCAlp.html?Mn=focuser

As you mentioned, Bortle 3 is pretty dark.

Of course, those Bortle 3 21.81 readings aren't 21.81 if there was any Moon at all in the sky, ...

Hy

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[Rich Klein]

That is a great video, Francesco.  How are the results impacted by narrowband filters?  Or more appropriately I guess, how, roughly, do narrowband filters impact SQM for emission nebulas, say?  

- Rich 

[Francesco Meschia]

Thanks, Rich! 

A narrowband filter, compared to a broadband one, reduces the number of photons that reach the sensor by the ratio between the two bandpass widths. So, a 3 nm filter would transmit only 1/100th of the light that, say, a L filter (300 nm bandpass window) would. Or, if you prefer, the sky radiance would appear to be five magnitudes lower than with the L filter.

Of course, this happens to all light, not just the undesired one. So there is an advantage only for line-emission target.

Francesco

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[Rajiv Subrahmanyam]

Thanks for the link Hy. I didn't know that the Bortle scale was so non-linear.

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[gary%mtha...@gtempaccount.com]

Thanks for this comparison. As a newbe living on Mt Hamilton in a Bartle 5 area I have been wondering what I might expect to be able to capture. This comparison really helps!